1. Field of the Invention
The present invention relates to a process for producing sodium persulfate. Sodium persulfate has been widely used in industrial process, for example, as a polymerization initiator for the production of polyvinyl chloride and polyacrylonitrile and as a treating agent for printed wiring boards.
2. Description of the Prior Art
As a general production method of sodium persulfate, the reaction between ammonium persulfate and sodium hydroxide in an aqueous solution has been known (U.S. Pat. No. 3,954,952). However, this process is uneconomical because the yield of sodium persulfate based on ammonium persulfate is low due to a large number of steps required. In addition, the concentration of sulfuric acid in the catholyte feed solution should be lowered to maintain a high solubility of ammonium sulfate to the catholyte feed solution, this increasing the electrolytic voltage, i.e., the unit power cost.
U.S. Pat. No. 4,144,144 discloses a direct electrolytic production of sodium persulfate using a neutral anolyte feed solution in the presence of ammonium ion. In this process, the mother liquor after removing crystallized sodium persulfate is mixed with a cathode product and recycled to an electrolytic step as the anolyte feed solution. Therefore, the electrolysis is conducted in the presence of sodium persulfate which participates nothing in the electrolysis, this increasing the electrolysis voltage and decreasing the current efficiency. In addition, since the resultant sodium persulfate crystals contain nitrogen in higher concentrations, a careful and thorough washing is necessary to purify sodium persulfate to an acceptable level for practical use.
An object of the present invention is to solve the above problems in the prior art and to provide a process for producing sodium persulfate in a low unit power cost and a reduced number of production steps.
After extensive study for solving the above problems, the inventors have found that sodium persulfate is more economically produced by electrolyzing an anolyte feed solution containing sodium sulfate, ammonium sulfate and sodium persulfate, reacting the resulting anode product with sodium hydroxide and crystallizing sodium persulfate by concentration, while recovering ammonia gas liberated from the crystallization step into a cathode product, followed by neutralizing the resulting cathode product with sodium hydroxide and/or ammonia and recycling a mixture of the neutralized solution with sodium sulfate recovered from a crystallization mother liquor as a part of the starting material for the anolyte feed solution.
Thus, the present invention provides a process for producing sodium persulfate, comprising (1) a step of electrolyzing a catholyte feed solution containing sulfuric acid and an anolyte feed solution containing sodium sulfate, ammonium sulfate and sodium persulfate, thereby obtaining a cathode product and an anode product; (2) a step of reacting the anode product with sodium hydroxide in a reaction-type crystallizer, thereby obtaining a reaction mixture; (3) a step of crystallizing sodium persulfate from the reaction mixture by concentration, thereby obtaining a sodium persulfate slurry; (4) a step of separating the sodium persulfate slurry to sodium persulfate crystals and a mother liquor, thereby recovering the sodium persulfate crystals; (5) a step of crystallizing sodium sulfate from the mother liquor, thereby obtaining a sodium sulfate slurry; (6) a step of separating sodium sulfate crystals from the sodium sulfate slurry; (7) a step of recovering ammonia gas liberated in the step (2) into the cathode product obtained in the step (1); (8) a step of neutralizing the resulting cathode product with sodium hydroxide and/or ammonia to obtain a neutralized cathode product; and (9) a step of recycling the neutralized cathode product and the sodium sulfate separated in the step (6) to the step (1) as a part of a starting material for the anolyte feed solution.
In the electrolysis step (1) of the process of the present invention, an aqueous solution containing, by weight, 5 to 18% sodium sulfate, 21 to 38% ammonium sulfate and 0.1 to 2% sodium persulfate is used as an anolyte feed solution. The sulfate ratio, sodium sulfate/ammonium sulfate, is preferably 0.1 to 0.9 by weight. When the sulfate ratio is less than 0.1, the available amount of sodium sulfate obtained in the separation step (6) is reduced to increase the unit material cost. A sulfate ratio higher than 0.9 increases the electrolytic voltage to increase the unit power cost. The anolyte feed solution may further contain 0.01 to 0.1% by weight of a known polarizer such as thiocyanate, cyanide, cyanate and fluoride. The catholyte feed solution is a 20 to 80% by weight aqueous solution of sulfuric acid.
The electrolytic cell usable in the present process is not specifically limited so long as it is structured to separate the anode from the cathode by a diaphragm, and a box electrolytic cell or a filter press electrolytic cell is preferably used. The diaphragm for the box electrolytic cell is made of an oxidation resistant material such as alumina. Ion-exchange membranes are preferably used as the diaphragm of the filter press electrolytic cell.
The anode is preferably made of platinum, although anodes made of a chemically resistant material such as carbon are usable. The cathode is preferably made of zirconium or lead, although cathodes made of a chemically resistant material such as stainless steel are usable. The anode current density is 40 to 120 A/dm2, preferably 60 to 80 A/dm2. A current density lower than 40 A/dm2 produces a poor current efficiency. A current density higher than 120 A/dm2 could be used, but uneconomical because a specific power supply equipment is needed due to a considerable heat generation at a bus bar.
The electrolytic cell is operated at 10 to 40xc2x0 C., preferably 25 to 35xc2x0 C. Temperatures lower than 10xc2x0 C. are detrimentally low because sodium sulfate, etc. begin to crystallize to make the process inoperable and an unnecessarily high electrolytic voltage is required. Temperatures exceeding 40xc2x0 C. are undesirably high because excessive decomposition of the resulting persulfate ion occurs to result in a low yield of sodium persulfate.
Then, the anode product from the electrolysis step (1) is introduced into a reaction-type crystallizer and reacted with an aqueous solution of sodium hydroxide in the step (2), followed by the step (3) where sodium persulfate is caused to crystallize from the reaction mixture by concentration. The reaction-type crystallizer is not specifically limited so long as it is operable under reduced pressure, and a reaction-type crystallizer equipped with an agitator, preferably a double propeller reaction-type crystallizer having a clarification zone is used. The reaction-type crystallizer so constructed facilitates the sampling of at least a part of the liquid therein in the step (3) for crystallizing sodium persulfate.
The crystallization of sodium persulfate in the reaction-type crystallizer is carried out at 15 to 60xc2x0 C., preferably 20 to 50xc2x0 C. When the temperature is lower than 15xc2x0 C., the reaction rate between the anolyte product and sodium hydroxide is low and the coexisting sodium sulfate is likely to crystallize to lower the purity of sodium persulfate crystals. At temperatures higher than 60xc2x0 C., excessive decomposition of the resulting sodium persulfate occurs to result in a low yield of sodium persulfate. The residence time in the reaction-type crystallizer depends on the desired particle size of sodium persulfate, and generally selected from the range of 1 to 10 hours. The residence time can be shorter than one hour if sodium persulfate with smaller particle size is desired.
Sodium hydroxide is added to the anode product solution introduced into the reaction-type crystallizer in an amount enough to displace at least proton and ammonium ion attributable to by-produced sulfuric acid, ammonium persulfate and ammonium sulfate present in the solution by sodium ion. Preferably, sodium hydroxide is added in an amount such that the liquid in the reaction-type crystallizer is adjusted to the pH range of 9 to 12. The rate of effusion of ammonia is low at a pH lower than 9 to increase the nitrogen content of the sodium persulfate crystals, and the persulfate ion is likely to decompose at a pH higher than 12 to reduce the yield of sodium persulfate. The pressure inside the reaction-type crystallizer is adjusted to a level which allows water to boil at the temperature range mentioned above. The liberated ammonia gas is recovered into the cathode product obtained in the electrolysis step (1), as described below.
The sodium persulfate slurry obtained in the crystallization step (3) is separated into sodium persulfate crystals and a mother liquor in the separation step (4) using a solid-liquid separator such as a centrifuging separator. The separated crystals are dried to the final product by a powder drier. The reaction step (2) and the crystallization step (3) may be operated in the same reaction-type crystallizer having a clarification zone.
The mother liquor is transferred into the reaction-type crystallizer of the step (2) or into the crystallization step (5) of sodium sulfate. The crystallization of sodium sulfate is preferably conducted by a cooling crystallization method where sodium sulfate precipitates as a hydrate in the step (5) and separated from the sodium sulfate slurry in the step (6), for example, by a common technique such as centrifuging separation. The mother liquid after separating the crystallized sodium sulfate is returned to the reaction-type crystallizer of the step (2). If the separation of sodium sulfate is omitted, sodium sulfate formed by the reaction with sodium hydroxide added in the step (2) will build up in the reaction-type crystallizer, and ultimately coprecipitate with sodium persulfate to reduce the purity of the sodium persulfate product. The crystallization of sodium sulfate is conducted in a cooling crystallizer equipped with a cooling means. If a double propeller crystallizer having a clarification zone is used in the step (2), the clarified liquid is treated to separate sodium sulfate.
Sodium sulfate is separated in an amount such that the concentration of sodium sulfate in the reaction-type crystallizer of the step (2) is maintained constant. Namely, sodium sulfate is removed in an amount corresponding to the total amount of the sulfate ion contained in the anode product to be fed into the reaction-type crystallization steps (2) and (3) and the sulfate ion formed during the operation of the reaction-type crystallization by the decomposition of persulfate ion. Namely, the amount of sodium sulfate to be removed can be determined by the total amount of the sulfate ion in the anode product measured by a common method such as titration and the amount of decomposed persulfate ion obtained from the material balance of the reaction-type crystallization steps (2) and (3). By regulating the feeding rate of the mother liquor to the cooling crystallizer so that sodium sulfate crystallizes in the determined amount, the desired amount of sodium sulfate can be precipitated and removed. The recovered hydrate of sodium sulfate is recycled as a part of the starting material for the anolyte feed solution as described below.
As described above, the precipitating amount of sodium sulfate depends on the feeding rate and the chemical composition of the starting solution to be fed into the cooling crystallizer. For example, in the cooling crystallization of a 30xc2x0 C. saturated solution containing, by weight, 35% sodium persulfate and 8% sodium sulfate at 18xc2x0 C., sodium sulfate decahydrate precipitates in an amount of about 8% by weight based on the starting saturated solution.
The cooling crystallization of the step (5) is conducted at 5 to 30xc2x0 C., preferably 15 to 25xc2x0 C. Sodium sulfate precipitate insufficiently at temperatures higher than 30xc2x0 C. to reduce the purity of the sodium persulfate product. Sodium persulfate coprecipitate with sodium sulfate at temperatures lower than 5xc2x0 C. to increase the content of sodium persulfate in sodium sulfate.
In the step (7), ammonia gas liberated from the reaction-type crystallizer of the step (2) is recovered into the cathode product obtained in the step (1), as described above. Sulfuric acid remaining in the cathode product after absorbing ammonia is neutralized with sodium sulfate and/or ammonia gas in the step (8). Then, sodium sulfate recovered in the step (6) and a desired amount of the polarizer are dissolved into the resulting neutralized solution in the step (9). The solution thus obtained is recycled as a starting material for the anolyte feed solution. To maintain the dissolution of sodium sulfate and the polarizer, the solution may be diluted with water.
In the continuous process of the present invention, the neutralization by sodium hydroxide is switched to the neutralization by ammonia gas and vice versa such that the sulfate ratio, sodium sulfate/ammonium sulfate, in the anolyte feed solution is regulated within the range of 0.1 to 0.9 by weight. Since ammonia and sodium sulfate are circulated in the present process, the amount of ammonia gas used in the neutralization corresponds to the loss of the ammonia in the recovery step (7).
A part of the anode product obtained in the electrolysis step (1) may be concentrated prior to the reaction with sodium hydroxide in the step (2) to increase the reaction rate between the anode product and sodium hydroxide in the reaction step (2). The degree of concentration can be increased by concentrating after mixing the anode product solution with the mother liquor after recovering sodium sulfate in the step (6). Since the mother liquor is a saturated solution at an operating. temperature (5 to 30xc2x0 C.) of the step (5), the degree of concentration can be increased as compared with when concentrating the as-obtained anode product solution.